1. Field of the Invention
The present invention relates to a telecommunications systems and in particular to a broadband signal space switching device having crosspoints comprising respective switching elements which are controllable by a decoder-controlled, crosspoint-associated memory cell.
2. Description of the Prior Art
Recent developments in telecommunications technology have led to service-integrated communications transmission and switching systems for narrowband and broadband communications services which provide light waveguides as the transmission media in the area of the subscriber lines by a way of which both the narrowband communications services such as, in particular, 64-kbit/s digital telephony and broadband communications services such as, in particular, 140 Mbit/s picture telephony are conducted, whereby, however, narrowband signal switching devices and broadband signal switching devices (preferably having shared control devices) are provided in juxtaposition in the switching stations. In this connection one may refer to the German Patent No. 24 21 002.
It is known in the context of a broadband signal time-division multiplex switching device whose crosspoints are respectively used in time-division multiplex for a plurality of connections, to connect two respective lines with the assistance of a gate which is switched on and off by a crosspoint-associated memory cell constructed of a bistable D-flip-flop. The crosspoint-associated memory cell, whose clock input is supplied with a corresponding clock signal, being driven in only one coordinate direction, in particular, at its D-input (Pfannschmidt, "Arbeitsgeschwindigkeitsgrenzen von Koppelnetzwerken fur Breitband-Digitalsignales", Dissertation, Braunschweig 1978, FIG. 6.7). In view of a time-division multiplex factor of about 4-8 attainable given the bit rate of 140 Mbit/s and in view of the involved circuit technology of required, however, pure space switching devices are presently preferred for switching broadband signals, the connections extended via the individual crosspoints being only spatially separated from one another.
A pure broadband signal space switching arrangement can be constructed of the crosspoint matrix in whose crosspoints the switching elements are respectively controlled by a crosspoint-associated holding memory cell that is only decoder-controlled (Pfannschmidt, op. cit. FIG. 6.4). The switching elements can thereby be respectively constructed as complementary metal-oxide-semiconductor (CMOS) transfer gates (CMOS transmission gates) (ISS'84 Conference Papers 23Cl, FIG. 9). The utilization of the simple CMOS transfer gate as a switching element, however, involves a load of its input line by the capacitance of its output line in the through-connected condition of the CMOS transfer gate, and this condition can cause signal delays. As a result of what are referred to as under-threshold effects (for example, in the form of stray currents or under-threshold currents) along the MOS transistors, however, crosstalk phenomena can occur at the switching elements that are actually located in the inhibited condition.
A similar case is also true of a broadband space switching device comprising crosspoints having switching elements respectively controllable by a crosspoint-associated memory cell, these switching elements being respectively formed with a CMOS inverter circuit comprising MOS transistors of the enhancement type which has its input side connected to the appertaining signal input line and has its output side leading to the appertaining signal output line, a p-channel depletion transistor having its control electrode connected to the output of the memory cell being inserted between the p-channel enhancement transistor and the appertaining feed voltage source and an n-channel depletion transistor having its control electrode connected to the complementary output of the memory cell being inserted between the n-channel enhancement transistor and the appertaining feed voltage source (ISS'84 Conference Papers 31C3, FIG. 14). As a consequence of a constant, at least weak conductivity of the two depletion transistors, such a construction of the switching elements leads to static dissipated power of a crosspoint, even in its inhibited condition, and also leads to the fact that the switching element in the through-connected condition is also loaded by switching elements which are in the inhibited condition and which are nonetheless weakly conductive. Furthermore, no full signal swings between the two values of feed potential are possible.